The First Positive Detection of Molecular Gas in a GRB Host Galaxy

We report on strong H 2 and CO absorption from gas within the host galaxy of gamma-ray burst (GRB) 080607. Analysis of our Keck/LRIS afterglow spectrum reveals a very large H i column density ( ) and strong metal-line absorption at z GRB = 3.0363 with a roughly solar metallicity. We detect a series of A − X bandheads from CO and estimate N (CO) = 10 16.5±0.3 cm −2 and T CO ex > 100 K. We argue that the high excitation temperature results from UV pumping of the CO gas by the GRB afterglow. Similarly, we observe H 2 absorption via the Lyman–Werner bands and estimate with –300 K. The afterglow photometry suggests an extinction law with R V ≈ 4 and A V ≈ 3.2 mag and requires the presence of a modest 2175 Å bump. Additionally, modeling of the Swift XRT X-ray spectrum confirms a large column density with N H = 10 22.58±0.04 cm -2 . Remarkably, this molecular gas has extinction properties, metallicity, and a CO/H 2 ratio comparable to those of translucent molecular clouds of the Milky Way, suggesting that star formation at high z proceeds in similar environments as today. However, the integrated dust-to-metals ratio is sub-Galactic, suggesting the dust is primarily associated with the molecular phase while the atomic gas has a much lower dust-to-gas ratio. Sightlines like GRB 080607 serve as powerful probes of nucleosynthesis and star-forming regions in the young universe and contribute to the population of "dark" GRB afterglows.